This invention relates to privacy transmission systems wherein signals indicative of source intelligence are enciphered and then transmitted through a communications channel having a known bandwidth constraint.
In a typical transmission system, a source of intelligence provides intelligence-bearing signals within a frequency range corresponding to a bandwidth constraint of a transmission channel. Analog output signals indicative of voice intelligence provided by a telephone are within the 3 kHz bandwidth constraint of the system. In privacy transmission, however, the output signals of the telephone must be enciphered. Transmission of these enciphered signals over the bandwidth limited channel often results in the introduction of significant distortion in the deciphered signal. The present invention relates to apparatus for additive enciphering and deciphering of signals indicative of source intelligence and for transmitting and receiving the enciphered signals over a narrow band channel, such as a standard telephone link, while minimizing the distortion produced thereby.
Additive enciphering is a form of enciphering in which a pseudo-random value is added directly or modulo some specified constant with the value of a pulse to determine a level or value of an enciphered pulse. Additive enciphering may be performed by analog or digital apparatus. In the case of analog enciphering, the analog level of the enciphered pulse is the value. In the case of a digital embodiment, an analog level is first converted into digital form, and a random number in digital form is added thereto in order to provide an enciphered and preferably binary output having a predetermined number of bits. Additive enciphering is distinguished from scrambling or mapping functions used for enciphering in which pulses are in a different order from the order in which they were generated. Scrambling functions are generally less desirable, however, since the introduction of a small error in the enciphered signal due to transmission lines characteristics may result in a large error in the subsequently desiphered signal.
Even in additive encipherment of signals, distortion introduced by transmission media, whether they be hard wire or atmospheric media, can produce unacceptable degradation of the deciphered signal. Therefore, it is desirable to provide an adaptive transversal filter in the receiving means. The adaptive transversal filter, sometimes referred to as an adaptive filter or adaptive equalizer, is a time domain signal equalizer which provides weights to incoming signals based upon the means square error between the received signal and a reference value. An example of one form of an adaptive transversal equalizer is illustrated in U.S. Pat. No. 3,798,560 issued Mar. 19, 1974, to Taylor. Another adaptive transversal filter and algorithm for adjusting weights therein are fully described in R. W. Lucky and H. R. Rudin, An Automatic Equalizer For General Purpose Communication Channels, Bell System R. W. Lucky, Automatic Equalization for Digital Communication, Bell System Technical Journal, April 1965, p 547. During a training mode at the beginning of a transmission weights of the adaptive filter are adjusted with each incoming signal.
In transversal filters currently in use, one sample of the incoming signal is taken in each symbol period. In these systems, the sampling time may be critical. The common practice of taking only one sample per symbol time of the adaptive equalizer may thus lead to achieving less than optimal response.
Further, since the receiving means must decipher the additively enciphered pulse train, the receiving means must also include a pseudo-random number generator. It is necessary to provide a means for synchronizing the pseudo-random number generator in receiving means with the pseudo-random number generator in the transmitting means.